Wearable ECG Monitor with Capacitively-Coupled Electrodes

A wearable ECG monitor was developed that utilizes capacitively-coupled electrodes, analog front-end amplification, a 16-bit ADC, and an ARM-based microprocessor with low-power Bluetooth communications. One objective was to study the efficacy of capacitively coupled electrodes while reducing their physical size. Our electrodes are typically 5x5 or 10x10 mm; a substantial reduction in electrode footprint compared to those reported in the literature. Physically smaller electrodes are essential for the high-fidelity recording of ECG signals, and for improved robustness of the capacitive electrode-to-body connection. The electrodes have particularly high specific capacitance, 1.5 µF/cm2, due to our ceramic nanolaminate dielectric. The dielectric is very thin and is deposited by atomic layer deposition. The dielectric also has a breakdown voltage greater than the battery voltage, essential for safety reasons. Bend test studies of electrodes fabricated on polyimide substrates have shown capacitance variations as a function of bending, and failure at small radii of curvature. Thus, flex substrate electrodes are being replaced with “rigid island” electrodes. The analog front-end uses an AD8232 chip, especially designed for processing ECG signals, at 1,100x gain. The ECG signal is filtered by both low and high software adjusted cut-offs, and a notch filter at 60Hz. On-board analysis of ECG R-peaks gives heart rate and heart rate variability parameters that are used to generate alarms. An objective of our studies is to determine if lower-level peaks in the ECG signal can be resolved using capacitively coupled electrodes. The 16-bit electronics are considered essential in this effort. Human subject testing per Binghamton University IRB Protocol 1370 will be reported for the electrodes and the monitor. Funding was provided by FlexTech/SEMI/AFRL contact 19-17 and New York’s Empire State Development Corporation.